The Reference Class Problem in Evolutionary Biology: Distinguishing Selection from Drift

Forthcoming: Chance in Evolution, edited by Charles Pence and Grant Ramsey, Chicago University Press.

Abstract: Evolutionary biology distinguishes differences in survival and reproduction rates due to selection from those due to drift. The distinction is usually thought to be founded in probabilistic facts: a difference in (say) two variants' average lifespans over some period of time that is due to selection is explained by differences in the probabilities relevant to survival; in the purest cases of drift, by contrast, the survival probabilities are equal and the difference in lifespans is a matter of chance. When there is a difference in actual average lifespans there is always a difference in causal histories, but in drift, the causal differences make no contribution to the relevant probabilities. What is the rationale for ignoring these differences in a probabilistic description of evolutionary change? This is evolutionary biology's version of philosophy of probability's reference class problem. Skeptical, relativist, and other deflationary answers beckon—perhaps it is something cultural or epistemological or psychological that decides what gets counted and what gets ignored in determining survival probabilities and so on. This paper uses the author's recent work on biological probabilities (Bigger than Chaos) and probabilistic explanation (Depth) to argue for a more objectivist answer: the causal factors that are counted are precisely those that make a difference to, and so are explanatorily relevant to, the frequencies of outcomes, such as surviving a predator encounter, that determine evolutionary change. Causal factors are ignored, then, just when they are explanatorily irrelevant to the episode of evolution to be explained.

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